Heterosexual transmission of HIV from both men to women and women to men is the principal mode of spread of the virus, but the exact mechanisms of transmission are not clearly understood. This study was designed to determine the 50% Macaque Infectious Dose (MID50) for cell-free SIVMne delivered by the intravaginal route in adult Macaca nemestrina. The intravaginal MID50 will be used as the basis for selecting virus challenge doses in candidate vaccine and antiviral drug trials. In addition to determining the MID50 for SIVMne, we simultaneously examined the kinetics of seroconversion in intravaginally infected animals by monitoring immunoglobulin levels and isotypes in several mucosal compartments and plasma and compared the antibody response with responses observed in intravenously inoculated macaques. Two adult female M. nemestrina were inoculated intravaginally with 104 TCID of uncloned SIVMne in June 1996. In addition, two females were inoculated intravenously with the same dose and virus. Blood was obtained every two weeks for the first 20 weeks and then monthly thereafter for virus culture, CD4+ cell assays, and antibody testing. Tears, saliva, vaginal washes, and rectal washes were obtained at the time of the blood draws. The two intravenously inoculated macaques became infected and developed a strong plasma antibody response to SIV. One of the intravaginally inoculated animals became infected and seroconverted while the other remained virus negative but showed a transient plasma antibody response. SIV-specific antibodies were detected in the tears, plasma, saliva, and vaginal washes of all the infected macaques. These four macaques were euthanized and histology was performed on the tissues at 52 weeks post-inoculation. Two additional macaques were inoculated intravaginally with 103 TCID in December 1997, with blood and mucosal sampling done on the same schedule as for the previous macaques. Both of these animals have seroconverted but virus has not been detected via viral co-cultures. PCR assays on lymph nodes and peripheral blood mononuclear cells from these macaques are pending.